Many accessory devices in a vehicle require relatively large amounts of current to operate. A vehicle's horn, air conditioner compressor clutch, blower motor and electric heaters are examples of accessory devices that require more electric current than most semiconductors are able to provide. The current that such devices require is usually provided to them using conventional, high-current relays, the actuations of which are controlled by a processor responsive to the processor detecting actuation of a control device for the accessory. These high current relays are preferably located in the engine compartment for safety but must nevertheless be controlled from the passenger compartment.
FIG. 1 depicts a prior art system 100 for controlling relays in a vehicle engine compartment from the passenger compartment. The system 100 comprises a passenger compartment-located body control module or “BCM” 102. The BCM 102 is comprised of a processor 104 (microcontroller or microprocessor). The processor 104 has conventional address/data/control lines 106, at least some of which are electrically coupled, i.e., interfaced, to an engine compartment-located power distribution center (PDC) 108 through a set of protective drivers 110, which pass through the vehicle's firewall 105, because they “drive” relay coils 113, which are inherently inductive.
The BCM 102 is electrically coupled to and receives electrical signals from various controls that are located in the passenger compartment for various electrical accessory devices located through-out the vehicle. Such inputs to a BCM 102 are well known to those of ordinary skill in the art and omitted from the figures for brevity.
When a driver or vehicle occupant wants to operate an electrically-powered accessory that requires more current that a processor or similar device can reliably provide by itself, the current for such a device is preferably provided to the device by the actuation of a high-current relay located on the engine compartment-located PDC 108. FIG. 1 thus shows the PDC 108 as having relays 112 that provide electrical battery power to various electrical loads 116. The relays 112 actuate a load, i.e., provide electric current to a load, responsive to electrical signals sent to the coils 113 of the relays 112 by the processor 104 located in the BCM 102, albeit through the current driver 110. The engine compartment-located relays on the PDC 108 are thus controlled by the processor 104, which is located in the BCM 102.
Unfortunately, the prior art requires each relay on a PDC 108 to have at least one, relay control wire connected to the processor 104. Since many vehicles require high reliability and maintainability, the PDC-located relays are also preferably “testable” by the processor 104 by being provided with sets of auxiliary diagnostic contacts 116, the opening and closure of which can be tested by the processor 104 using dedicated diagnostic test wires 118, which also extend between the relays 112 and processor 104. The multiple test wires 118 and relay control wires 120 that extend between the protective driver 110 and relay coils 113 make the connection between a BCM 102 and PDC 108 costly and actually decrease a vehicle's reliability. Reducing the number of wires and/or connectors that extend between a passenger compartment-located body control module and an engine compartment-located power distribution center would be an improvement over the prior art.